With the introduction of glutaraldehyde preservation of biological tissue, and in particular porcine bioprosthetic heart valves, it has become possible to: (a) overcome the poor performance of early formaldehyde-preserved implanted tissue valves; (b) discontinue the use of homograft valves; and (c) avoid the undesirable use of anticoagulants required to prevent thromboembolism associated with the use of non-bioprosthetic (mechanical) heart valves, especially in children. Not unlike other similarly important discoveries, however it appears that the glutaraldehyde-preserved bioprosthesis has created its own dilemma.
Although the relatively biologically inert glutaraldehyde-preserved valves of Carpentier and others have demonstrated excellent long-term durability in most instances, serious drawbacks such as tissue-fatigue and a propensity toward calcification have plagued their continued use. Moreover, it was initially contemplated that children and adolescents would be among those deriving the greatest benefit from the glutaraldehyde-preserved bioprosthetic heart valves since the anticoagulants required with mechanical prostheses could be eliminated. Results from an increasing number of recent clinical studies indicate that severe calcification of these tissues with relatively short-term failure is prevalent among children and adolescents. Thus, despite their long-term durability and overall reduced incidence of complications, these glutaraldehyde-preserved valves have been deemed by some to be unsuitable for use in children.
Calcification of tissue remains a mystery for the most part; however, it has previously been shown that various factors including calcium metabolism diseases, age, diet, degeneration of tissue components such as collagen, and turbulance are all involved to a certain extent. Recently, the occurrence of a specific calcium-binding amino acid, laid down after implantation of glutaraldehyde-preserved porcine xenografts, has been demonstrated; and it has been postulated to play a role in calcification. While calcification has been accompanied by degradative changes in the glutaraldehyde-treated collagen fibers of the implanted tissue, it remains unclear whether the dystrophic calcification is a cause or the result of tissue degeneration. Nevertheless, there has been a continued effort to elucidate the source of the calcification problem with implanted tissue, with the hope that a remedy would be soon to follow. Heretofore, neither the source or cause of calcification in biological implants nor the appropriate measures to prevent or reduce calcification in biological implants have been ascertained.
In accordance with the present invention, we have determined an underlying cause of calcification with biological implants, and in particular with glutaraldehyde-preserved valvular bioprostheses. Furthermore, we have concurrently developed procedures which effectively reduce or mitigate calcification of implanted biological tissue.
One of the underlying causes of calcification in valvular bioprostheses, noted for the first time in our immediate studies, is the presence of phosphate in contact with the tissue prior to implantation in amounts which sustain calcification after implantation. The levels of phosphate normally found in shipping media, such as balanced salt solutions (Hanks', etc.), plasma, and 0.01 to 0.10M phosphate-buffered-saline (PBS) conventionally used in glutaraldehyde-fixing solutions, all sustained calcification in tissue to varying degrees. Heretofore, the deleterious effects of phosphate in contact with biological implant tissue have not been appreciated, and accordingly researchers, clinicians and manufacturers alike have been unaware of the undesirable consequences caused by their treatment of these implants with phosphate-containing solutions; particularly because phosphate solutions such as Hanks', PBS, and glutaraldehyde-PBS are so commonly used and even highly recommended. It would therefore be understandable why unwittingly the interchangeability of PBS and bicarbonate buffers (having similar buffering capacities and pH ranges) might have been recommended for tissue storage resulting in sporadic substitution for phosphate-containing media. Moreover, in some instances, even bicarbonate-buffered tissue storage media contained high levels of phosphate. Since the deleterious consequences of maintaining tissue implants in contact with phosphate were unknown, there was no deliberate intent on the part of clinicians or manufacturers of bioprostheses to avoid contacting the tissue with phosphate solutions.
In accordance with the present invention, we have developed processes which effectively reduce calcification of implanted biological tissue. These processes advantageously reduce the tendency of bioprostheses toward calcification and overcome some of the problems associated with the durability of xenograft heart valves.